In recent years, a so-called four-wheel drive hybrid automobile using an electric motor as a driving source is suggested. Additionally, among four-wheel drive hybrid electric automobiles, an electric automobile which drives right and left front wheels with a liquid fuel engine and drives right and left rear wheels with a direct-current motor is known (for example, refer to Patent Document 1).
In the four-wheel drive hybrid electric automobile in the example of Patent Document 1, the driving force of an alternating-current (AC) motor is transmitted to a differential gear of a rear wheel via a speed reducer and an electromagnetic clutch, and the driving force of the AC motor is distributed to right and left rear wheels by this differential gear.
However, in such a four-wheel drive hybrid electric automobile, machine parts such as the differential gear and the electromagnetic clutch exist, and reduction of fuel cost is difficult due to the weight of the machine parts. Therefore, further improvement in fuel efficiency by reduction of the weight of the machine parts is required.
Thus, there is suggested a structure which drives the right and left rear wheels (for example, refer to Patent Document 2) in which switched reluctance motors (hereinafter referred to as “SR motors”) are directly arranged in the wheel hubs of the right and left rear wheels, respectively, not via a differential gear and an electromagnetic clutch, the output shafts of the SR motors are connected to the brake discs fixed to the wheels, and the driving force of the SR motors is transmitted to the wheels via the brake discs.
On the other hand, generally an automobile transmits rotation of an engine shaft which is an output portion of a liquid fuel engine to the wheels, and travels by the rotation of the wheels.
Then, in order to cope with traveling modes, such as the start of a vehicle on an upward slope, that is, traveling at low speed with high torque, and traveling on a highway, that is, traveling at high speed with low torque, the rotation of the engine shaft is decelerated or accelerated (shifted) by a transmission, and is transmitted to the wheels. That is, the automobile travels by rotating a driving shaft by the rotation of the decelerated or accelerated engine shaft, and rotating the wheels connected to the driving shaft. In particular, since it is necessary to rotate the wheels with large torque in order to start the vehicle from a stopped state, the transmission requires a mechanism which significantly decelerates the rotation of the engine shaft.
Moreover, it is necessary for the vehicle not only to travel forward but also to enter a parking lot or leave the parking lot, or reverse for a change in direction or the like. Therefore, the transmission requires a mechanism which converts the direction of rotation of the engine shaft into rotation in a direction opposite to the direction when traveling forward, transmits the rotation converted into the rotation in the opposite direction to the driving shaft, and rotates the driving shaft in the opposite direction.
FIG. 7 shows an example of a transmission attached to a liquid fuel engine of an automobile. This drawing shows an internal structure of a transmission loaded on an automobile (a so-called FF vehicle) in which a liquid fuel engine is arranged at the front of a vehicle to drive right and left front wheels. An engine shaft 30 of the liquid fuel engine is inserted into the transmission 3, and is rotatably arranged inside the transmission 3.
In the engine shaft 30 to be inserted into the transmission 3, a reverse driving gear 32a, a first driving gear 33a, a second driving gear 34a, a third driving gear 35a, and a fourth driving gear 36a are integrally formed sequentially toward the side (left side on the plane of the sheet) far from the liquid fuel engine from the side (right side on the plane of the sheet) near the liquid fuel engine. Additionally, in the first to fourth driving gears (33a to 36a), the number of teeth z1 of the first driving gear 33a is the smallest, and the number of teeth is set so as to become large in order of the number of teeth z2 of the second driving gear 34a, the number of teeth z3 of the third driving gear 35a, and the number of teeth z4 of the fourth driving gear 36a. 
A main shaft 31 which is a driven shaft is rotatably arranged inside the transmission 3, similarly to the engine shaft 30, at a position parallel to the engine shaft 30 which is a main driving shaft. In the main shaft 31, a final driving gear 37a, a reverse driven gear 32b, a first driven gear 33b, a second driven gear 34b, a third driven gear 35b, and a fourth driven gear 36b are provided sequentially toward the side (left side on the plane of the sheet) far from the liquid fuel engine from the side (right side on the plane of the sheet) near the liquid fuel engine. The final driving gear 37a and the reverse driven gear 32b are formed integrally with the main shaft 31.
Meanwhile, a recess-shaped serration 31a which extends in an axial direction is formed at regular pitches in a circumferential direction on the surface of a portion (the left side on the plane of the sheet) in which the final driving gear 37a and reverse driven gear 32b of the main shaft 31 are not formed. The first driven gear 33b, the second driven gear 34b, the third driven gear 35b, and the fourth driven gear 36b are movable in the axial direction along the above-described recess shape of the main shaft 31, the movement of these gears in the circumferential direction is prevented by the main shaft 31. Additionally, the movement of the first to fourth driven shafts (32b to 36b) in the axial direction is controlled by a command from the vehicle.
In the first to fourth driven gears (33b to 36b), the number of teeth Z1 of the first driven gear 33b is the greatest, and the number of teeth is set so as to become small in order of the number of teeth Z2 of the second driven gear 34b, the number of teeth Z3 of the third driven gear 35b, and the number of teeth Z4 of the fourth driven gear 36b. 
The first driven gear 33b meshes with the first driving gear 33a, and the first driving gear 33a and first driving gear 33b mesh with each other so as to form a first gear pair 33. Similarly, the second driven gear 34b meshes with the second driving gear 34a, and the second driving gear 34a and the second driving gear 34b mesh with each other so as to form a second gear pair 34, the third driven gear 35b meshes with the third driving gear 35a, and the third driving gear 35a and the second driving gear 35b mesh with each other so as to form a third gear pair 35, a fourth driven gear 36b meshes with the fourth driving gear 36a, and the fourth driving gear 36a and the second driving gear 36b mesh with each other so as to form a fourth gear pair 36.
Here, as described above, although the first to fourth driven gears (33b to 36b) are moved in the axial direction of the main shaft 31 by a command from the vehicle, only one gear pair of the first to fourth gear pairs (33 to 36) is controlled so as to be selected during this movement. Here, the transmission ratio r1 of the first gear pair is Z1/z1, the transmission ratio r2 of the second gear pair is Z2/z2, the transmission ratio r3 of the third gear pair is Z3/z3, and the transmission ratio r4 of the fourth gear pair is Z4/z4. When one gear pair is selected by a command from the vehicle, the main shaft 31 rotates in the transmission ratio (r1 to r4) of a selected gear pair, and the rotation of the engine shaft 30 is decelerated or accelerated.
The driving shaft 39 is rotatably arranged at the transmission 3 at a position parallel to the main shaft 31 similarly to the main shaft 31, a front right wheel driving shaft 39a protrudes from the transmission 3 in one direction (right direction on the plane of the sheet), and a front left wheel driven shaft 39b protrudes from the transmission 3 in the other direction (left direction on the plane of the sheet 9. A differential gear 38 is provided at an axial center portion of the driving shaft 39 inside the transmission 3, and a final driven gear 37b is connected to an outer circumferential portion of the differential gear 38.
The final driven gear 37b meshes with the final driving gear 37a formed integrally with the main shaft 31, and a final gear pair 37 is formed by the final driving gear 37a and the final driving gear 37b. Here, the number of teeth of the final driving gear 37a is defined as zf, the number of teeth of the final driven gear 37b is defined as Zf, and the reduction ratio rf of the final gear pair 37 is Zf/zf. In addition, in general vehicles, the reduction ratio rf is defined as about 3 to about 6.
As described above, the rotation of the engine shaft 30 is shifted by one gear pair selected from the first to fourth gear pairs (33 to 36), and then decelerated by the final gear pair 37, and the driving shaft 39 is rotated. Accordingly, the transmission ratio R of the driving shaft 39 to the engine shaft 30 becomes R1=r1×rf, R2=r2×rf, R3=r3×fr, and R4=r4×rf, R1 becomes the greatest, R2, R3, and R4 become small in this order, and R4 becomes the smallest (R1>R2>R3>R4).
Incidentally, in the case of the start from stop where a low speed and a high torque are required, the transmission ratio R1 is selected. When the number of revolutions of the engine shaft 30 is defined as Ne, the number of revolutions Nd of the driving shaft 39 in the case of the transmission ratio R1 becomes Nd=Ne/R1, and torque is increased while the number of revolutions becomes small. In addition, in general vehicles, often, the transmission ratio R1 is set to about 11 to 15, the transmission ratio R2 is set to about 6 to 9, the transmission ratio R3 is set to about 4 to 7, and the transmission ratio R4 is set to 3 to 5.
Next, the structure and operation of the transmission 3 for moving a vehicle backward will be described. As described above, the reverse driving gear 32a is formed integrally with the engine shaft 30 at the end of the engine shaft 30 inside the transmission 3 on the side (right side on the plane of the sheet) near the liquid fuel engine 2. Additionally, in the main shaft 31, the reverse driven gear 32b is formed integrally with the main shaft 31. Although the reverse driving gear 32a is arranged at the same position as the reverse driven gear 32b in the axial direction, the reverse driving gear 32a and reverse driven gear 32b are set so as not to directly mesh with each other.
The transmission 3 is equipped with a reverse idle gear 32c which is movable in the axial direction of the engine shaft 30 and the main shaft 31 which are arranged parallel to each other. The reverse idle gear 32c meshes with both the reverse driving gear 32a and the reverse driven gear 32b. In a case where the vehicle is advancing, the reverse idle gear 32c is arranged at a position where the reverse idle gear does not mesh with either the reverse driving gear 32a or the reverse driven gear 32b, and the reverse idle gear moves in the axial direction on a command of moving the vehicle backward from the vehicle, the reverse idle gear is arranged at a position where the reverse idle gear meshes with both the reverse driving gear 32a and the reverse driven gear 32b. When the reverse idle gear 32c meshes with both the reverse driving gear 32a and the reverse driven gear 32b, the rotation of the reverse driving gear 32a is transmitted to the reverse driven gear 32b via the reverse idle gear 32c, and the reverse driven gear 32b rotates.
The rotation of the above-described first to fourth driving gears (33a to 36a) is directly transmitted to the first to fourth driven gears (33b to 36b), whereas similarly to the rotation of the first to fourth driving gears (33a to 36a), the rotation of the reverse driving gear 32a formed integrally with the engine shaft 30 is transmitted to the reverse driven gear 32b via the reverse idle gear 33c. Therefore, the rotation of the reverse idle gear 33c becomes reverse to the direction of rotation of the first to fourth driven gears (33b to 36b), and the main shaft 31 rotates in a direction reverse to the direction at the time of an advance on a command of moving the vehicle backward from the vehicle. The rotation of the main shaft 31 is transmitted to the driving shaft 39 via the final driven gear 37b from the final driving gear 37a, and rotates the driving shaft 39 in a direction reverse to the direction at the time of an advance.
The number of teeth of the reverse driving gear 32a is defined as zr, and the number of teeth of the reverse driven gear 32b is defined as Zr, and the transmission ratio of the reverse gear group 32 composed of the reverse driving gear 32a, the reverse driven gear 32b, and the reverse idle gear 32c is rr=Zr/zr. Accordingly, the transmission ratio Rr at the time of a reverse of the vehicle becomes Rr=rr×rf. In addition, since the start from a stopped state is performed at the time of a reverse, in order to rotate the driving shaft 39 at a low speed with a high torque, the transmission ratio Rr at the time of a reverse of the vehicle is a value close to the transmission ratio R1 at the time of an advance of the vehicle (Rr≅R1). In addition, in general vehicles, the transmission ratio Rr is often about 10 to 14.
In the automobile using the liquid fuel engine as a driving source in this way, there is a need for traveling forward at a wide range of speed from low speed to high speed, and reverse traveling, and as shown in FIG. 7, various combinations of gears are indispensable to the configuration (for example, refer to Patent Document 3). Therefore, irrespective of the fact that the light weight of a transmission is required in reduction in the size and weight of an automobile, there are limits to significant reduction in the size and weight of the transmission.    [Patent Document 1] Japanese Patent Application, First Publication No. 2006-288006    [Patent Document 2] Japanese Patent Application, First Publication No. 2002-305861    [Patent Document 3] Japanese Patent Application, First Publication No. 2007-147057